1. Field of the Invention
The present invention relates to a developing device that is used in an image forming apparatus such as a copying machine or a laser printer that employs an electrostatic recording method or an electrophotographic method in which an electrostatic image that has been formed on an image-bearing member is developed by using a developer that includes toner and carrier.
2. Description of the Related Art
In the related art, in an image forming apparatus such as a copying machine, an electrostatic latent image that has been formed on a photoconductor drum, which is an image-bearing member, is developed by a developing device in such a manner as to convert the electrostatic latent image into a visible image.
Such a developing device is configured to cause a latent image to be visualized as a toner image by supplying toner to a photoconductor drum, and a developing device in which a two-component developer that is a mixture of non-magnetic toner and magnetic carrier is used as a developer is frequently used in addition to a developing device in which a mono-component developer that contains magnetic toner is used as a developer. In a developing device that uses a two-component developer, the charge amount of toner has good stability, and thus, a color image that has good color tone can be formed. Therefore, such a developing device may be used in color image forming apparatuses.
An example of the above-mentioned developing device is disclosed in Japanese Patent Laid-Open No. 11-143231 (Patent Document 1), and a representative figure thereof is illustrated in FIG. 5.
In a developing device 102, a developing sleeve 120 that includes a cylindrical magnet 121, which is non-rotatably arranged in an opening of a developer container 110 that contains a two-component developer, is disposed in such a manner as to face a photoconductor drum 1. The photoconductor drum 1 rotates in the direction of a corresponding one of arrows in FIG. 5, and the developing sleeve 120 rotates in a direction opposite to the direction in which the photoconductor drum 1 rotates at a facing portion.
The developer, which is contained in the developer container 110, is sucked in and held on the developing sleeve 120 at a position (pumping position) Q on a surface of the developing sleeve 120 that corresponds to the position of a pumping magnetic pole S1 of the magnet 121 by the influence of the pumping magnetic pole S1. Then, the developer reaches a development portion after the layer thickness of the developer has been controlled by a developing blade 122, forms a magnetic brush in the development portion by the influence of a developing magnetic pole N1, and develops a latent image on the photoconductor drum 1.
The developer, whose toner density has been reduced through development, is separated from the developing sleeve 120 and deposited at a position (developer depositing position) P on the surface of the developing sleeve 120 between a releasing magnetic pole S2 and the pumping magnetic pole S1 where the magnetic flux density is low. As described above, the developer is sucked in and held on the developing sleeve 120, from which the developer has been separated, again at the pumping position Q.
In the developer container 110, a developing chamber 125 in which a first stirring conveying member 123 is disposed is formed below the developing sleeve 120, and a stirring chamber 126 in which a second stirring conveying member 124 is disposed is formed in such a manner that a partitioning wall 140 is interposed between the developing chamber 125 and the stirring chamber 126. Each of the first and second stirring conveying members 123 and 124 is a screw-type stirring conveying member, and such a screw-type stirring conveying member generally includes a helical screw blade.
Circulation of the developer in the developing device 102 will be described with reference to FIG. 6. The developing chamber 125 from which the developer is supplied to the developing sleeve 120 and the stirring chamber 126 that is arranged next to the developing chamber 125 in the horizontal direction are separated from each other by the partitioning wall 140 that has end portions 115 and 116 that are open. As indicated by arrows in FIG. 6, the developer is supplied to the developing sleeve 120 while the developer is made to circulate between the developing chamber 125 and the stirring chamber 126 by the first and second stirring conveying members 123 and 124. The developer that has passed through a development region of the photoconductor drum 1 while being carried by the developing sleeve 120 is separated from the developing sleeve 120 and collected so as to be placed in the developing chamber 125 as indicated by dashed arrows in FIG. 6.
In this case, if the developer that has been separated from the developing sleeve 120 and that has a low toner density is not sufficiently stirred by the first stirring conveying member 123 and supplied to the developing sleeve 120 again, the developer develops a latent image on the photoconductor drum 1 while having a low toner density. This causes image density to be unstable, and undesirable effects such as inconsistencies in density are likely to occur.
Accordingly, as a configuration for suppressing the occurrence of the inconsistencies in density due to a developer that has been separated from a developing sleeve and that has a low toner density, a developing device that employs a function separation system such as that disclosed in Japanese Patent Laid-open No. 2012-42737 (Patent Document 2) has been proposed. An exemplary embodiment of the developing device disclosed in Patent Document 2, which employs the function separation system, is illustrated in FIG. 7.
A developing device that employs the function separation system is a developing device that employs a developer circulation system in which a developer is supplied from a developing chamber, in which a first conveying screw is disposed, to a developing sleeve, and in which the developer after completion of development is collected so as to be placed in a stirring chamber in which a second conveying screw is disposed. By employing the function separation system, a developer whose toner density has been reduced as a result of being used for development can be prevented from being supplied to a developing sleeve again while not being sufficiently stirred, and the occurrence of a problem, such as inconsistencies in density as mentioned above, can be suppressed.
However, in a developing device that employs the function separation system such as that disclosed in Patent Document 2, there has been a problem in that uneven distribution of the developer level occurs. This uneven distribution occurs as a result of the developer being collected from the developing sleeve and placed in the stirring chamber.
Circulation of a developer in such a developing device that employs the function separation system will be described with reference to FIG. 8 while being compared with FIG. 6. As indicated by arrows in FIG. 8, the developer is supplied to a developing sleeve 120 while the developer is made to circulate between a developing chamber 125 and a stirring chamber 126 by conveying screws. This configuration is the same as that of the developing device of FIG. 6, which does not employ the function separation system. However, in the developing device of FIG. 8, which employs the function separation system, the developer that has passed through a development region of a photoconductor while being carried by the developing sleeve 120 is separated from the developing sleeve 120 and then collected so as to be placed, not in the developing chamber 125, but in the stirring chamber 126, as indicated by dashed arrows in FIG. 8. This is a characteristic of a developing device that employs function separation system.
In the developing device, which is illustrated in FIG. 6 and which does not employs the function separation system, the developer that has been supplied to the developing sleeve 120 from the developing chamber 125 returns the developing chamber 125, and thus, the distribution of developer is relatively uniform. However, in the developing device, which is illustrated in FIG. 8 and which employs the function separation system, the developer that has been supplied to the developing sleeve 120 from the developing chamber 125 is collected so as to be placed in the stirring chamber 126, and thus, in the developing chamber 125, the surface angle of the developer is likely to become inclined downward toward a downstream side. As a result, the amount of the developer on the downstream side of the developing chamber 125 that is to be supplied to the developing sleeve 120 is reduced, which in turn may result in coating failure.
Regarding a decrease in the level of a developer in a developer container of a developing device that employs the function separation system, such as that described above, for example, a developing device that is disclosed in Japanese Patent Laid-Open No. 11-84874 (Patent Document 3) has been proposed. Patent Document 3 proposes that, in a developing device, particularly a vertical-stirring-type developing device that employs the function separation system and in which a developing chamber is formed above a stirring chamber, unevenness in developer distribution is corrected by changing the pitch or the diameter of the blade of a conveying screw, which is disposed in the developing chamber.
However, a method for making the unevenness uniform during developer distribution in a developing chamber by changing the pitch or the diameter of the blade of a conveying screw, which is disposed in the developing chamber, such as that described in Patent document 3, has the following problem.
For example, although reducing the pitch of the blade of a conveying screw, which is disposed in a developing chamber, can increase the developer level, as the pitch of the blade is reduced, the inclination angle of the blade with respect to a screw shaft increases. When the inclination angle of the blade of the conveying screw with respect to the screw shaft is small, a force that is applied to the developer by the blade as a result of rotation of the conveying screw increases not only in a direction parallel to the screw shaft, but also in a direction perpendicular to the screw shaft. This force affects the ability of the blade to supply the developer to a developing sleeve. However, when the inclination angle of the blade of the conveying screw with respect to the screw shaft is large, the force that is applied to the developer by the blade as a result of rotation of the conveying screw concentrates in the direction perpendicular to the screw shaft, and thus, the ability of the blade to supply the developer to the developing sleeve decreases.
As described above, there is a concern that, if the pitch of the blade in the developing chamber is reduced, or if the diameter of the blade is changed, the ability of the blade to supply the developer to the developing sleeve also changes in response to these changes. Thus, there is a concern that, even if the developer level can be increased by changing the pitch or the diameter of the blade, at the same time, the ability of the blade to supply the developer to the developing sleeve also changes, and as a result, the advantageous effect of improving coating of the developing sleeve is limited.
Accordingly, a configuration in which the pitch of the blade in the developing chamber is reduced or the diameter of the blade is changed in an area outside a developer carrying area of the developing sleeve in a direction parallel to the rotation axis of the developing sleeve may also be considered. However, although a decrease in the level of the developer in the area outside the developer carrying area of the developing sleeve can be suppressed with the above-described configuration, the configuration is not sufficiently effective for the level of the developer in the developer carrying area of the developing sleeve.